BNG in Practice: What the First Year of Mandatory Biodiversity Net Gain Has Shown

Mandatory Biodiversity Net Gain became a legal requirement for major development in England on 12 February 2024 and for minor development on 2 April 2024, under Schedule 7A of the Town and Country Planning Act 1990 (inserted by the Environment Act 2021). Every qualifying planning permission must now deliver a minimum 10% net gain in biodiversity value, measured using the statutory Biodiversity Metric.

The first year of mandatory BNG made the practical reality clear. Some of the early fears have not materialised. Some unexpected challenges have emerged. And the system is still evolving as local authorities, developers, ecologists, and Natural England find their way through the framework: on 6 August 2026 new exemptions and a revised delivery hierarchy for minor development came into force.

The legal requirement is deceptively simple: a minimum 10% net gain in biodiversity value, with significant on-site enhancements and registered off-site gains maintained for at least 30 years. But the detail matters.

Biodiversity value is measured using the statutory biodiversity metric (published by Defra on 29 November 2023, replacing the earlier Biodiversity Metric 4.0). The metric calculates a site's biodiversity value in "biodiversity units" based on habitat type, condition, area, and strategic significance. The applicant must demonstrate that the post-development biodiversity value exceeds the pre-development baseline by at least 10%.

The gain can be delivered through three mechanisms, applied in a strict hierarchy: on-site habitat creation or enhancement (preferred), off-site habitat creation through registered biodiversity gain sites, or purchase of statutory biodiversity credits from Natural England (last resort). The hierarchy is important: for major development, applicants must demonstrate that on-site delivery has been maximised before resorting to off-site solutions. For minor development where the planning application was made on or after 6 August 2026, registered off-site gains carry the same preference as on-site habitat creation or enhancement, and statutory credits remain the last resort.

NPPF policy N1(1)(b) sets the wider plan-making context: development plans should identify opportunities for "the conservation, enhancement and recovery of landscapes, sensitive waterbodies, habitats and priority or threatened species", including through habitat restoration and the creation and strengthening of ecological networks. The NPPF leaves the net gain figure to the statute: under policy N2(3), gains above it should only be required where up-to-date development plan policies for specific site allocations set them out. BNG sits within this broader ecological policy context.

The statutory biodiversity metric is a detailed calculation tool, and getting it right requires genuine ecological expertise. Our analysis of BNG submissions shows that the most common errors are in baseline habitat classification and condition assessment. Classifying a habitat incorrectly (for example, recording modified grassland as "other neutral grassland") can significantly change the number of units and the scale of the gain required.

Condition assessment is particularly subjective. The metric provides condition assessment criteria for each habitat type, but applying them consistently requires professional ecological judgement. Two ecologists can assess the same grassland and reach different condition scores, which directly affects the baseline biodiversity value.

The early concern that BNG would sterilise large areas of development land has generally not materialised. For the majority of residential schemes, 10% net gain can be achieved through a combination of habitat creation in public open space, enhanced boundary planting, species-rich grassland in SuDS features, and improved management of retained habitats.

The key is early integration into the design process. Schemes that bolt BNG on at the end, after the layout is fixed and the landscaping is designed, struggle to achieve on-site delivery. Schemes that consider biodiversity from masterplan stage find that the habitat creation often aligns with other requirements: green infrastructure, SuDS, amenity space, and landscape buffers.

For schemes that cannot achieve 10% on-site, the off-site market is still maturing. Registered biodiversity gain sites are being established, but availability is patchy, particularly for specific habitat types in specific locations. The strategic significance multiplier in the metric means that off-site gains in the right location deliver more units per hectare than those in less strategically important areas.

Statutory biodiversity credits, available from Natural England as the last resort, are deliberately expensive. They are priced to incentivise on-site and off-site delivery rather than credit purchase. At current prices, credits for most habitat types cost significantly more per unit than bespoke off-site solutions.

The biggest practical challenge in the first year has not been ecological. It has been administrative. Many local planning authorities are still building capacity to validate and assess BNG submissions. Ecologist resource within planning departments is limited, and the technical complexity of metric calculations means that non-specialist case officers struggle to assess whether a submission is adequate.

This has led to delays in validation, extended determination periods, and inconsistent application of the requirements between authorities. Some authorities have published detailed BNG validation checklists and guidance; others are still working out their approach.

The Chartered Institute of Ecology and Environmental Management (CIEEM) has published guidance on the role of ecologists in BNG, emphasising that metric calculations should be carried out by competent ecologists with relevant field experience. CIEEM's guidance on preliminary ecological appraisal (2017) and ecological impact assessment (2018, updated September 2024) provides the professional framework for BNG work.

In practice, outside the small sites metric (which GOV.UK says can be completed without an ecologist), a BNG metric calculation submitted by someone without ecological competence is at risk of rejection: the local planning authority reviews submitted calculations and may reject them if it does not believe a competent person completed them. The metric is a tool, and it requires professional judgement to apply correctly.

Not all development requires BNG. Key exemptions include:

Permitted development (development granted planning permission by a development order, so no planning application is needed) is exempt. Householder applications (extensions, outbuildings) are exempt. For planning applications made on or after 6 August 2026, development on a site of 0.2 hectares or below (measured within the red line boundary) is exempt, as is wholly temporary development permitted for five years or less. Development impacting habitat below the de minimis threshold (less than 25 square metres of area habitat and less than 5 metres of linear habitat) is exempt. None of these three exemptions applies where on-site priority habitat is affected. The exemption for self-build and custom housebuilding no longer applies to planning applications made on or after 6 August 2026.

Small developments (for example, 1-9 dwellings on a site under 1 hectare, or commercial development creating less than 1,000 square metres of floor space on a site under 1 hectare) can use the simplified small sites metric rather than the full statutory metric, which reduces the assessment burden. It cannot be used where priority habitats (other than some hedgerows and arable field margins), protected sites, or European protected species are present.

Based on the first year of mandatory BNG, several practical recommendations emerge:

Commission a Preliminary Ecological Appraisal early. Understanding your baseline habitat before you design the scheme is essential. A PEA will identify the habitats present, their approximate condition, and the likely scale of BNG required.

Integrate BNG into the masterplan. Do not treat biodiversity as an afterthought. Work with your ecologist and landscape architect from the outset to identify where habitat creation can be delivered within the scheme layout.

Allow for 30-year management. Significant on-site enhancements and off-site gains must be maintained for at least 30 years, secured by a planning condition, a Section 106 agreement, or a conservation covenant (off-site gains by agreement or covenant only). The Biodiversity Gain Plan, approved by the planning authority before development begins, must describe how significant on-site enhancements will be maintained and monitored. Factor the management costs into your viability assessment and ensure the management mechanism (management company, local authority adoption, or conservation body) is agreed early.

Check local authority requirements. Each authority is developing its own BNG validation requirements and supplementary guidance. Check what is required before submitting your application.

Mandatory BNG is a significant change to the planning system, but it is workable. The first year has shown that with early ecological input, integrated design, and professional metric calculations, most developments can achieve 10% net gain without fundamentally compromising scheme viability.